What does "brought aboard" mean?

To bring something aboard, or to have it brought aboard, is to have the thing brought into some kind of conveyance; particularly a plane or ship. For a car the more common phrase would be 'brought it with' or 'put it in'. The same idea also applies to 'come aboard': you 'come aboard' a plane or ship, you 'get in' a car.

One can go either way with a train. You can 'get on' a train or 'get aboard' a train.